MOS technology (particularly CMOS technology) is currently of limited suitability for mixed signal (analog) designs. Thus, most microelectronic devices which operate with real-world signals (such as video, voice, feedback control, etc.) signals typically require both an analog circuit or multiple analog ICs along with the less costly digital IC devices.
Some (pseudo-analog) devices have been proposed for CMOS and BiCMOS devices. However pseudo-analog devices do not permit low power compressed designs therefore defeating the advantages present in a single chip MOS solution.
One of the substantial difficulties present in developing mixed signal circuits on MOS technology is the fabrication of passive components (resistors, capacitors, inductors, etc).
One of the challenging tasks faced by mixed signal designs, is providing a single chip solution. One limiting factor is to fabricate accurate passive components on-chip.
Various techniques are used to implement passive components in the monolithic Integrated circuits.
On-chip resistors typically are fabricated by base diffusion, emitter diffusion, ion-implantation or by thin-film deposition. In the MOS technology, the popular resistor implementations being diffused or polysilicon or well resistors or thin-film deposition or MOS devices themselves used as resistors. The resistor value that can be achieved by the above mentioned means can normally vary from 50 ohms to 50 Ohms, which is technology and process dependent.
On-chip capacitors typically are fabricated by poly-poly, poly-metal, metal-metal process or using MOS transistor capacitance and junction capacitance. Normally an extra layer of poly layer is added to provide efficient capacitor structure.
On-chip inductance typically is realized by synthesizing an inductive reactance with an active circuit. Passive inductance can be implemented on-chip using transmission lines. They are superior as they introduce less noise, consume less power, and have a wider bandwidth and linear operating range.
The crucial parameters to be considered for passive components are their tolerance, voltage coefficient and temperature coefficient. Each technique has its pros and cons, but none of them tend to result in accurate values. For example, a common technique 100 used for correcting the value of passive components which are resistors is by laser trimming as is shown in FIG. 1. In this figure, a resistor 102 extends between two terminals labeled "A" and "B" on an integrated circuit (IC) chip. Conductive lines 104 and 106 extend from the terminals "A" and "B" to other circuitry (not shown) on the IC chip 102. The resistor 102 is essentially a thin layer of resistive (partially conductive) material extending from the one terminal "A" to the other terminal "B", and is typically in the form of a rectangle, as shown, having a length (between the terminals) and a width transverse to the length.
To trim (adjust) the resistance value of the resistor 102, its resistance is measured, then the physical structure of the resistor 102 is altered by a process known as laser trimming, wherein a laser beam ablates the resistor material. Large resistance value corrections can be made by directing a laser beam (not shown) into the resistor material from a side thereof, partially across the width of the resistor, then along the length of the resistor, as indicated by the L-shaped notch 110. Smaller resistance value corrections can be made by directing the laser beam (not shown) into the resistor material from a side thereof and partially across the width of the resistor as indicated by the notch 112.
A drawback to the laser trimming technique is that it is not feasible for CMOS mass production devices. Also, it is of limited value in establishing a desired capacitance value for a capacitor or a desired inductance value for an inductor.